This invention relates generally to power generation combustors and more specifically to combustor baffles.
Industrial power generation gas turbine engines include a compressor for compressing air that is mixed with fuel and ignited in a combustor for generating combustion gases. The combustion gases flow to a turbine that extracts energy for driving a shaft to power the compressor and produces output power for powering an electrical generator, for example. The turbine is typically operated for extended periods of time at a relatively high base load for powering the generator to produce electrical power to a utility grid, for example. Exhaust emissions from the combustion gases are therefore a concern and are subjected to mandated limits.
Gas turbine combustors are being developed that employ lean premixed combustion to reduce emissions of gases such as NOx (nitrogen oxides). One such combustor comprises a plurality of burners attached to a single combustion chamber. Each burner includes a flow tube with a centrally disposed fuel nozzle comprising a center hub which supports fuel injectors and swirl vanes. During operation, fuel is injected through the fuel injectors and is mixed with the swirling air in the flow tube, and a flame is produced at the exit of the burner. The combustion flame is stabilized by a combination of bluffbody recirculation behind the center hub and swirl-induced recirculation. Because of the lean stoichiometry, lean premixed combustion achieves lower flame temperature and thus produces lower NOx emissions.
These premixed systems, however, are susceptible to an unpredictable phenomena commonly referred to as "flashback." Flashback is caused by any of a number of events including ignition of impurities in fuel or ignition during mode switching when the flames are in a transient phase. When flashback occurs, a flame enters zones or cavities of the combustor chamber that are not designed to contain flames. One such zone where a flashback is of concern is the region between the fuel nozzles up against the endcover. The recirculating air flow pattern in this region and the possible presence of fuel makes it susceptible to flashback. A flashback in this area can result in a loss of combustion control and can additionally cause heating and melting of combustor parts like fuel nozzles, for example, that are not designed to withstand excessive heating.
Furthermore, the ever increasing demand for turbines with improved thermodynamic efficiency has resulted in higher turbine inlet temperatures. The higher turbine inlet temperatures create a corresponding increase in compressor discharge temperatures and pressure ratio. The combination of these conditions make the autoignition of fuel more likely.
Accordingly, there is a need in the art for improved recirculation within the endcover region to reduce the occurrence of flashback.